(a) Show that ∼ defines an equivalence relation on the chessboard.
(b) How many equivalence classes are there, and what are their sizes?
(c) Choose equivalence class representatives for the classes.

Here are my thoughts...
a) would I assume ~ if i+j not equal to i'+j' and show this is not the case?
b) I'm not entirely sure but I would assume that as the canonical projection map is surjective and that there are 64 ways to write i+j that there are 64 equivalence classes of size 2, then again I'm well prepared to believe I've missed the point!
c) As a set of class representatives is a subset of X which contains exactly one element from each equivalence class could the class representative just be either the i's or the j's

Luckily this is over the computer so if people tell me I'm completly wrong you won't see me blushing!!

Very often, when approaching a new problem, is it useful to consider several examples. What are all the squares for which the sum of coordinates is 4? 5? This will help you find out almost everything about equivalence classes.

To show that ∼ is an equivalence relation (as well as to show anything in mathematics) one should start with definitions. Once you know the definitions of an equivalence relation and related notions, you need to know what it means to prove the statements involved. Those statements have the form "For all x, ...", so you need to know what it means to prove them.

You said there were 15 classes. you need to explicitly state one member of each class to represent that class. for example, partitioning the integers according to parity is an equivalence relation. i can choose 0 to be the class representative of the even integers while 1 can be that of the odd integers.